So I changed the battery in the truck. Opened the door and found the dome light on, then proceeded to start the truck. Ran fine. I pulled the light switch and the dash lights came on, then cut out... I now have no headlights, cab lights, instrument lights or door locks. I tested battery terminals and grounds, all good. Checked fuses for continuity, all good. Checked switch.... have power at the orange and black wire but nothing at tan/white wire. Traced back to fuse box. The fuse is not blown but checked power. No voltage at fuse box for #4 fuse. I jumped power from orange/black wire to white/tan wire while engaging the switch and got light and door locks in cab then pulled switch all the way and jumped to yellow and black to get headlights and all lights..... ideas on where to go from here? Already tried a new switch. Happy to be apart of the forum!

 

Hey Ryger, welcome! Hmm, was going to say at this point you've done all the testing, so just change the switch. But you did that!
I know the likely hood that a new switch is bad right out of the box is pretty high these days, unfortunately. But to have the exact same unusual problem with both? Very low probability I'd say.

So this happened right after changing the battery? Was it just the battery getting old, or did some event trigger it's early demise?
When you checked the grounds did you check both sides of the radiator core support and anything else attached to the body? I think it's a long shot, even though grounds always seem to be at the heart of odd issues. But in your case I would think a bad ground would effect more circuits than just the headlights.
But those core support grounds are for lots of stuff, including the computer and other VIP components.

Maybe triple-check them while we're still scratching our collective heads?

Good luck.

Paul

 

Oh, and I did not look in your profile, so please post up your model year, engine, body type and any other pertinent details about your truck. Just in case someone with the same year had the same issue.

Thanks

Paul

 

Thank you for the reply Paul, I will definitely check that out and reply back with an update. Thanks for the welcome as well! I love these older f-series trucks. This is the second one I have owned, both had 460’s. This one is a 1990 f250 Ext Cab 4x4 7.5l A4OD. I will be sure to update my profile with this information.

Sorry I couldn’t get it to sit right side up

 

Just remembered another common issue on Fords of just about any description. And that's a general deterioration of the headlight switch connector from heat. Is your switch still exposed where you can check it out easily? Do you remember any discoloring of wires from heat, or perhaps some of that blue/green ugly stuff from moisture? Any wires scuffed bare?
That last I assume you would have dealt with had you seen them, but maybe check the connector and it's strain relief really close. Maybe even some corrosion building up on the contacts themselves.
A few years ago I had to re-do the harness at the switch on a '95 Mustang due to some burned wires. I remember being surprised because I thought the whole "overheated light switch" thing was an old issue from the sixties and seventies. Shocked to see it again on a nineties car.
I wonder if it was always a natural progression from Ford refusing to use headlight relays in the system? Must be...
Imagine my surprise (again!) when I recently found out they were not using relays even right up to my '93 truck. I just assumed they'd finally seen the light and followed the truly almost universal practice with the other OE's of using headlight relays.

You don't happen to have upgraded headlight bulbs by any chance, do you?

Is fuse #4 the dash lights? If so there will not be any power to the fuse until the switch is turned on and the rheostat turned up enough to illuminate them normally. Since you don't have any lights without that jumper wire, there is likely no power coming out of the switch through the Blue w/red tracer wire for all the dash illumination.

Anyway, all this is basically going back to the switch and/or the wires. It might be worth the investment (hah! that's a funny word to use here) to try another switch. The new parts are such crap, I'd try a different brand too.
But check the wires first. If any look compromised at all it's worth a trip to the junkyard to cut a few inches of harness and a new connector to splice into yours. That's what I did to the Mustang and it worked amazing after that.

Then install headlight relays if you don't already have them.

Paul

 

Hey Paul Just an update. I checked all grounds everything seems fine I did something that you may or may not approve of. I spliced into the tan/white wire before the #4 fuse and ran it to the battery and bam. Problem fixed I assume there was a break/short in the harness for the tan/white wire. I appreciate your help with this Paul and do plan on installing a relay for this light switch.

 

Cool. Hey, a fix is a fix, whether it's MacGyvered or Rube Goldberged or whatever. As long as it's safe.
Or as we like to say, "hey, it's only temporary... Unless it works!"

Glad you out-clevered it.
Now on to the next bit of truck-drama, whatever that ends up being.

Paul